In a high-tensile steel machine component receiving high-cycle load, it is necessary to reduce a risk of breakage troubles of components caused by an internal fatigue fracture starting from internal defects such as inclusions. It is conceivable that a size of the inclusion to be the starting point and an existence probability of the inclusion at a high-stress portion have large influence on the internal fatigue fracture. Conventionally, a method forecasting a fatigue limit starting from the inclusion has been formulated, and it is possible to estimate the fatigue limit by an estimated formula as stated above under a uniform stress (refer to Non-Patent Literature 1).
However, there is no effective method to evaluate the fracture performance as the machine component such as a spring having a complicated stress distribution such as effects of a shot peening and a shear stress. Accordingly, the present situation is that a fatigue design is performed based on an existing fatigue design diagram and so on based on an actual performance of materials.